As known, an axial fan generally comprises a hub and a plurality of blades which extend substantially in a radial direction from the hub.
The hub is rotatable about an axis and is connected to an electric motor for receiving a rotary motion by way of a transmission system.
The blades are provided with an airfoil, so that the rotation effect imparted by the motor, generates a pressure difference between the extrados and intrados of the blades. In turn, the pressure difference produces an air flow in a direction substantially parallel to the hub axis.
The air flow rate provided in axial motion depends on various factors, comprising mainly the rotation speed, the shape of the airfoil and the pitch angle of the blades.
It is known that, given a certain rotation speed, the incidence angle (i.e. the angle between the velocity vector of the air and the chord of the blade) is determined by the pitch angle and cannot exceed a critical threshold or stalling angle. In axial fans for industrial use, the pitch angle of the blades is normally between −4° and +30° (the pitch angle is usually measured using an inclinometer placed on the extrados of the blade at its distal end and oriented perpendicular to a radial direction).
Below the critical threshold, the air flow along the surface of the blades is laminar and allows to correctly exploit the curvature of the intrados and extrados of the blade to get lift. The turbulence is confined downstream from the reunification point of the flows lapping the extrados and the intrados, i.e. substantially downstream of the trailing edge of the blade.
If, instead, the incidence angle exceeds the critical threshold (stalling angle), the flows lapping the extrados and intrados fail to rejoin uniformly, are detached from the surface of the blade, and cause vortices downstream of the detachment point. The detachment takes place usually from peripheral areas of the blade, where the tangential speed is higher.
The vortices cause a loss of lift and, consequently, a decline of the fan efficiency. In practice, the flow rate set in motion does not increase or even decreases in response to a corresponding increment in the energy absorbed by the motor which drives the fan.
It is possible to design the blades of an axial fan so that the efficiency is greater for higher pitch angles of air and high speed, in part by limiting the risk of exceeding the critical threshold and trigger the formation of vortices. To this improvement, however, corresponds a reduced efficiency for pitch angles and/or lower speeds. Conversely, blades designed to have high efficiency at low pitch angles and at low speeds are totally unsatisfactory for higher angles and speeds, both for the low efficiency, and for greater ease to stall.
In axial fans for industrial use, in fact, the conditions of peripheral speed and pitch angles may vary in a substantial way. The axial fans for industrial use have normally, in fact, diameters ranging from about 1 m to about 12 m but the peripheral speeds can reach about 75 m/s. The pitch angles, instead, can vary in a range of about 30°-40°, as already noted. The working point may thus vary significantly and axial fans known are able to ensure sufficient efficiency only in a narrow range of operating conditions, contrary to what would be desirable. The difficulty of achieving satisfactory performance in a wider range of operating conditions is largely dependent on the separate peculiarities of axial fans for industrial use, particularly on the large size. A blade of said axial fans, in fact, measuring several meters in the radial direction, and therefore the speed difference between the distal end and the proximal end is very high, enough to bring the peripheral portions of the blades to stall conditions while the radially innermost portions still have a relatively abundant margin, but that cannot be exploited.